The study was conducted using stratified random sampling. Three levels of stratification were used in the sample: firm sector, firm size, and geographic region.
Industry stratification was designed in the following way: the universe was stratified into seven manufacturing industries and one "other" manufacturing category, - two services categories, retail and IT, and one "other" services category. Each of the manufacturing categories had a target of 160 interviews; the "other" manufacturing category and the three services categories all had targets of 120 interviews.
Size stratification was defined following the standardized definition for the Enterprise Surveys: small (5 to 19 employees), medium (20 to 99 employees), and large (more than 99 employees). For stratification purposes, the number of employees was defined on the basis of reported permanent full-time workers. This seems to be an appropriate definition of the labor force since seasonal/casual/part-time employment is not a common practice, except in the sectors of construction and agriculture.
Regional stratification was defined in eight locations (city and the surrounding business area): Mexico City, Estado de Mexico (MAMC), Guadalajara, Monterrey, Puebla, Monclova, Veracruz, and Leon.
Ciudad Juarez and Coahuila, which were included in the 2006 round of the Enterprise Surveys, were omitted in 2010 due to security concerns.
For Mexico, two sample frames were used. The first was supplied by the World Bank and consists of enterprises interviewed in Mexico 2006. The World Bank required that attempts should be made to re-interview establishments responding to the Mexico 2006 survey where they were within the selected geographical locations and met eligibility criteria. That sample is referred to as the Panel. The second sample frame was produced from the 2009 Economic Census of INEGI (Instituto Nacional de Geografía e Informática), the national bureau of statistics.
INEGI's database uses the SCIAN 2007 classification for economic activities while the Enterprise Surveys are based on the ISIC classification. Therefore, a conversion between the two classifications was made.
The two sample frames were then used for the selection of a sample with the aim of obtaining interviews with 1,600 establishments with five or more employees.
The quality of the frame was assessed at the outset of the project through visits to a random subset of firms and local contractor knowledge. The sample frame was not immune from the typical problems found in establishment surveys: positive rates of non-eligibility, repetition, non-existent units, etc. In addition, the sample frame contained no telephone/fax numbers so the local contractor had to screen the contacts by visiting them. Due to response rate and ineligibility issues, additional sample had to be extracted by the World Bank in order to obtain enough eligible contacts and meet the sample targets.
Given the impact that non-eligible units included in the sample universe may have on the results, adjustments may be needed when computing the appropriate weights for individual observations. The percentage of confirmed non-eligible units as a proportion of the total number of sampled establishments contacted for the survey was 12.55% (1079 out of 8600).
The number of realized interviews per contacted establishment was 0.17. The estimate is based on the total number of firms contacted including ineligible establishments. This number is the result of two factors: explicit refusals to participate in the survey, as reflected by the rate of rejection (which includes rejections of the screener and the main survey) and the quality of the sample frame, as represented by the presence of ineligible units. The number of rejections per contact was 0.29.
Complete information regarding the sampling methodology, sample frame, weights, response rates, and implementation can be found in "Description of Mexico ES 2010 Implementation" in external resources.
For some units it was impossible to determine eligibility because the contact was not successfully completed. Consequently, different assumptions as to their eligibility result in different universe cells' adjustments and in different sampling weights. Three sets of assumptions were considered:
a - Strict assumption: eligible establishments are only those for which it was possible to directly determine eligibility. The resulting weights, which include adjustments applied to panel firms, are in the variable w_strict_panadj.
b - Median assumption: eligible establishments are those for which it was possible to directly determine eligibility and those that rejected the screener questionnaire or an answering machine or fax was the only response. The resulting weights are included in the variable w_median_panadj.
c - Weak assumption: in addition to the establishments included in points a and b, all establishments for which it was not possible to contact or that refused the screening questionnaire are assumed eligible. This definition includes as eligible establishments with dead or out of service phone lines, establishments that never answered the phone, and establishments with incorrect addresses for which it was impossible to find a new address. Under the weak assumption only observed non-eligible units are excluded from universe projections. The resulting weights are included in the variable w_weak_panadj.